NANO LETTERS
Tuning the Reactivity of Energetic Nanoparticles by Creation of a Core−Shell Nanostructure
2005 Vol. 5, No. 7 1357-1360
A. Prakash,† A. V. McCormick,‡ and M. R. Zachariah*,† Department of Mechanical Engineering and Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742, and Department of Chemical Engineering and Material Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455 Received April 2, 2005; Revised Manuscript Received June 14, 2005
ABSTRACT This article presents a novel method for tuning the reactivity of nanoenergetic materials by coating a strong oxidizer nanoparticle (potassium permanganate; ∼150 nm) with a layer of a relatively mild oxidizer (iron oxide). The measured reactivity for a nano-Al/composite oxidizer could be varied by more than a factor of 10, as measured by the pressurization rate in a closed vessel (psi/µs), by changing the coating thickness of the iron oxide. The composite oxidizer nanoparticles were synthesized by a new aerosol approach in which the nonwetting interaction between iron oxide and molten potassium permanganate aids the phase segregation of a nanocomposite droplet into a core−shell structure.
Thermite metastable intermolecular composites (MIC) are a fascinating class of energy-intensive materials and have been a subject of extensive research over the past decade.1-3 Such materials are composed of an intimate mixture of nanoparticles of two components, fuel and oxidizer. Because the goal is to enhance the reactivity, the use of nanoscale material reduces the mass transport limitations between the fuel and oxidizer and the reaction becomes kinetically controlled.4 Aluminum nanoparticles are invariably used as fuel, whereas there are a host of metal oxide nanoparticles that are used as oxidizers for the MIC. Thermodynamic calculations of adiabatic flame temperatures and reaction enthalpy help us choose the possible MIC components from a large number of possible nanothermite combinations. Of the numerous thermodynamically possible MIC formulations,5 the most widely used are nano-Al (fuel) combined with MoO3, CuO, and Fe2O3 oxidizers. We recently reported a new MIC formulation of nano-Al combined with KMnO4 nanoparticles,6 the reactivity of which is about 2 orders of magnitude higher than the traditional formulations. Although there has been considerable success in formulating new composites with enhanced energy-release rates,1-4,6-8 the subject of achieving a precise control over the reactivity of nanothermites is an opportunity for further research. There have been some studies done on the size-dependent reactivity9,10 of nanoparticles as a means to control reactivity. In * Corresponding author. E-mail:
[email protected]. † University of Maryland. ‡ University of Minnesota. 10.1021/nl0506251 CCC: $30.25 Published on Web 06/25/2005
© 2005 American Chemical Society
one of our earlier works,11 we reported a method of chargeenhanced particle assembly to realize enhanced energyrelease rates. An MIC mixture of Al/KMnO4, although very reactive, has a very poor shelf life compared to the other traditional MIC combinations, the reason being the strong oxidizing nature of KMnO4. In such an MIC, the oxidizer slowly converts all of the aluminum to its oxide, reducing the shelf life. In this contribution, we report a novel technique to moderate the reactivity of nanoenergetic materials. We describe the synthesis of composite oxidizer nanoparticles with a core containing the strong oxidizer (potassium permanganate) and the shell of variable thickness of a relatively mild oxidizer (iron oxide). Using a composite oxidizer (potassium permanganate coated with iron oxide) as described in this paper, we are able to tune the reactivity over a relatively large dynamic range by changing the thickness of the less-reactive oxidizer. An additional benefit is that the structure has an improved shelf life and also reduces the sensitivity of the nanoenergetic mixture to any unintended initiations. The composite oxidizer nanoparticles were synthesized by a new single-step, two-temperature aerosol spray-pyrolysis method, the schematic of which is shown in Figure 1. A key to the synthesis of the desired microstructure is employing the difference in the characteristic temperatures of the two components. The difference between the thermal decomposition temperature of iron nitrate (
